The present invention is directed to a multilayer system for protecting components exposed to severe environmental and thermal conditions such as the hostile environment present in gas turbine engines.
A major limitation in the efficiency and emission of current gas turbines is the temperature capability (strength and durability) of metallic structural components (blades, nozzles and combustor liners) in the engine hot section. Although ceramic thermal barrier coatings are used to insulate metallic components, thereby allowing the use of higher gas temperatures, the metallic component remains a weak link. Such components must allow for the possibility of coating loss from spallation or erosion.
Silicon-containing ceramics are ideal materials for high temperature structural applications such as heat exchangers, advanced gas turbine engines, and advanced internal combustion engines. They have excellent oxidation resistance in clean oxidizing environments due to the formation of a slow-growing silica scale (SiO2). However, durability in high temperature environments containing molten salts, water vapor or a reducing atmosphere can limit their effectiveness. Molten salts react with silica scale to form liquid silicates. Oxygen readily diffuses through liquid silicates and rapidly oxidizes the substrate. High water vapor levels lead to hydrated silica species (Si(OH)x) and subsequent evaporation of protective scale. Complex combustion atmospheres containing oxidizing and reducing gases form SiO2 and reduce it to SiO(g). In situations with low partial pressure of oxidant, direct formation of SiO(g) occurs. All of these reactions can potentially limit the formation of a protective silica scale and thus lead to accelerated or catastrophic degradation.
Examples of silicon-containing ceramics are SiC fiber-reinforced SiC ceramic matrix composites (SiC/SiC CMC""s), SiC fiber-reinforced Si3N4 matrix composites (SiC/Si3N4 CMCs), carbon reinforced SiC ceramic matrix composites (C/SiC CMCs), monolithic silicon carbide and monolithic silicon nitride. A primary problem Si-containing ceramics face is rapid recession in combustion environments due to the volatilization of silica scale via reaction with water vapor, a major product of combustion. Therefore, use of silicon-containing ceramic components in the hot section of advanced gas turbine engines requires development of a reliable method to protect the ceramic from environmental attack. One approach in overcoming these potential environmental limitations is to apply a barrier coating which is environmentally stable in molten salts, water vapor and/or reducing atmosphere.
An early environmental barrier coating system (EBC) consisted of two layers, a mullite (3Al2O3.2SiO2) coat and a yttria-stabilized zirconia (YSZ) top coat. The mullite coat provided bonding, while the YSZ coat provided protection from water vapor. Mullite has a good coefficient of thermal expansion match and chemical compatibility with Si-based ceramics. However, the relatively high silica activity of mullite and the resulting selective volatilization of silica cause its rapid recession in water vapor. This EBC provided protection from water vapor for a few hundred hours at 1300xc2x0 C. During longer exposures, however, water vapor penetrated through cracks in the mullite and attacked the Si-containing substrate, leading to coating delamination.
Another EBC with improved performance was developed as part of a NASA High Speed Research-Enabling Propulsion Materials (HSR-EPM) Program in joint research by NASA, GE, and Pratt and Whitney. The EBC consisted of three layers: a silicon bond coat, an intermediate coat consisting of mullite or mullite and barium strontium aluminosilicate (BSAS), and a BSAS top coat. The mullite, mullite and BSAS, and BSAS layers were applied by a modified plasma spray process developed at the NASA Glenn Research Center as disclosed in U.S. Pat. No. 5,391,404, which is incorporated by reference herein in its entirety. The EBC was applied to SiC/SiC CMC combustor liners used in three Solar Turbine Centaur 50s gas turbine engines. The combined operation of the three engines resulted in the accumulation of tens of thousands of hours without failure at a maximum combustor lining temperature of about 1250xc2x0 C. A drawback of this BSAS-top coat EBC is that when applied to the solar turbine SiC/SiC liners it suffered from substantial BSAS recession after engine testing.
FIG. 1 of EP 1142850 shows an EBC which employs a YSZ topcoat 18, a YSZ-containing intermediate layer 24 between the topcoat and a Si-containing substrate 12, and a BSAS layer 22 between the YSZ-containing intermediate layer 24 and the substrate. The inventor of the present application has found that when BSAS and YSZ react, an undesirable low melting glass results. This problem was not recognized in the EP ""850 disclosure as is apparent by the close proximity of BSAS layer 22 and YSZ layer 24. EP ""850 discloses that intermediate YSZ-containing layer 24 can includes sublayers in which an inner sublayer in contact with the BSAS layer 22 contains one of BSAS, mullite or alumina and an outer sublayer in contact with the YSZ top coat consists essentially of YSZ. This again is disadvantageous in that it can position the outer YSZ sublayer in contact with an inner sublayer containing BSAS.
EP ""850 also discloses compositionally grading layer 24 using YSZ and one of BSAS, mullite or alumina. The EBC of EP ""850 will have BSAS-mullite contact when the mullite/YSZ graded layer is used. The inventor has found that mullite-BSAS reaction can become a serious durability issue in long-term exposures (over several hundred hours) as the reaction has the potential to produce a reaction product with a melting point as low as 1300xc2x0 C. Thus, the inventor has found that it is desirable to avoid the mullite-BSAS contact especially at outer layers where the temperature is higher. Another disadvantage is that the BSAS layer 22 adds 125 to 500 xcexcm of thickness to the EBC. A thick EBC has increased interlayer stress which may result in delamination. Therefore, the BSAS layer 22 at best presents a risk of delamination and at worst is deleterious to the EBC upon reaction of BSAS and YSZ or reaction of BSAS and mullite.
Current EBCs fail by delamination and spallation along a xe2x80x9cweak linkxe2x80x9d. The stress caused by the YSZ layer accelerates the failure. A key source for the creation of this xe2x80x9cweak linkxe2x80x9d is environmental/chemical degradation. Key material properties for long life EBCs should include environmental/chemical stability, low CTE, low modulus, sinter resistance, low thermal conductivity, and phase stability. Multilayer systems containing a YSZ outer layer and a mullite or mullite and alkaline earth metal aluminosilicate-containing intermediate layer, are potentially effective EBC systems, but there is a need to improve their performance by prolonging life or increasing the capacity to withstand higher operating temperatures.
The present invention is directed to a multilayer article which includes a substrate comprising a compound selected from the group consisting of a ceramic compound, a Si-containing metal alloy and combinations thereof. The multilayer article also includes an outer layer and a plurality of intermediate layers located between the outer layer and the substrate. The outer layer comprises fully or partially stabilized zirconia (ZrO2), preferably yttria stabilized zirconia, although rare earth elements besides yttria may be used as stabilizers. Intermediate layers are located between the outer layer and the substrate, one of which comprises a mullite (3Al2O3.2SiO2)-containing layer comprising 1) mullite or 2) mullite and an alkaline earth metal aluminosilicate. The mullite (no BSAS) intermediate layer is desirable when the multilayer article is used at temperatures of 1300xc2x0 C. and above for extended periods of time. One intermediate layer is an (outer) chemical barrier layer located between the mullite-containing layer and the outer layer. Yet another intermediate layer is an optional (inner) chemical barrier layer located between the mullite-containing layer and a bond layer or, if no bond layer is used, between the mullite-containing layer and the substrate.
The outer chemical barrier layer comprises a compound selected from the group consisting of mullite, hafnia (HfO2), hafnium silicate (e.g., HfSiO4), rare earth silicate (e.g., at least one of RE2SiO5 and RE2Si2O7 where RE is Sc or Yb), and combinations thereof, and, in particular, is hafnia, hafnium silicate or rare earth silicate. The outer chemical barrier layer is preferably located between an intermediate layer and the outer layer, more particularly, in contact with the outer layer and even more particularly is contiguous with both the outer layer and the mullite-containing layer.
The outer chemical barrier layer may be compositionally graded and consists essentially of a compound selected from the group consisting of mullite, hafnia (HfO2), hafnium silicate (e.g., HfSiO4 ), rare earth silicate (e.g., at least one of RE2SiO5 and RE2Si2O7 where RE is Sc or Yb), and combinations thereof in contact with the mullite-containing layer and consists essentially of stabilized zirconia in contact with the outer layer. The chemical barrier layer has a decreasing concentration of at least one of the mullite, hafnia (HfO2), hafnium silicate (e.g., HfSiO4 ) and rare earth silicate (e.g., at least one of RE2SiO5 and RE2Si2O7 where RE is Sc or Yb), and an increasing concentration of the stabilized zirconia, in a direction toward the outer layer.
The combination of the inventive outer chemical barrier layer composition and lack of a layer consisting essentially of BSAS between the mullite-containing layer and the outer chemical barrier layer, provides the multilayer article with very good performance. The outer chemical barrier layer advantageously inhibits or prevents the reaction between the outer layer and the mullite-containing layer. For example, when the outer layer includes YSZ and the mullite-containing layer includes BSAS, the chemical barrier layer prevents the formation of low melting glass resulting from YSZ and BSAS reaction. This chemical barrier effect is especially beneficial when the multilayer article is subjected to high temperatures (i.e., temperatures at 1300xc2x0 C. and above), where the interdiffusion between adjacent layers increases. In addition, by not employing a BSAS layer between the mullite containing layer and the outer chemical barrier layer, the thickness of the multilayer article can be substantially reduced. The thicker the EBC, the more likely that stresses will develop leading to delamination. In particular, a thickness of a portion of the multilayer article between an inner surface of the outer chemical barrier layer and an outer surface of the substrate ranges from 25 to 250 xcexcm and, more specifically, may be less than 200 xcexcm.
The inner mullite chemical barrier layer may be used between the mullite and glass ceramic intermediate layer and the silicon bond layer (or between the mullite and glass ceramic intermediate layer and the substrate when no bond layer is used) to prevent detrimental glass ceramic (e.g. BSAS, CAS, MAS)-silica reaction.
The inventive multilayer system includes two types of optional bond layers. The bond layer (e.g., bond coat) may be located between the mullite-containing layer and the substrate, preferably in contact with the substrate. A silicon-containing bond layer is preferred when the temperature at the bond layer is below the melting point of silicon. Alternatively, a bond layer which comprises a silicon-containing metal alloy having a melting point above the melting point of silicon may be used, such as Moxe2x80x94Si alloy and Nbxe2x80x94Si alloy. Suitable bond layer compositions would be apparent to those skilled in the art in view of this disclosure.
Turning to more specific features of the multilayer article of the invention, the alkaline earth metal aluminosilicate is preferably a compound selected from the group consisting of barium strontium aluminosilicate or BSAS (xBaO.(1-x)SrO.Al2O3.SiO2) where 0xe2x89xa6xxe2x89xa61, calcium aluminosilicate or CAS (CaO.Al2O3.2SiO2), magnesium aluminosilicate, also referred to as MAS or cordierite (2MgO.2Al2O3.5SiO2), and combinations thereof. An especially suitable mullite-containing layer comprises mullite and BSAS.
Although the outer layer may be referred to as a top coat, it need not be a coating per se. Also, other layers may be placed on top of the outer layer (i.e., further from the substrate than the outer layer). It should be understood that terms such as xe2x80x9cupper, lower, top, bottomxe2x80x9d and the like are used in this disclosure for purposes of illustration and should not be used to limit the invention, since these relative terms depend upon the orientation of the substrate. The intermediate layers are typically layers that are applied to the substrate or to a layer(s) on the article. However, some substrate materials, such as mullite matrix-containing materials, inherently form an intermediate layer (e.g., mullite). Whether applied or inherent, both constitute intermediate layers as these terms are used in this disclosure. The intermediate layers are defined herein as being located between the outer layer and a bond (e.g., Si-containing) layer on the substrate or, if no bond layer is used, between the outer layer and the substrate.
The substrate may be a ceramic compound, a Si-containing metal alloy, or both. The ceramic of the substrate may be a Si-containing ceramic or oxide ceramic with or without Si. The substrate comprises one of the following compounds: a Si-containing ceramic, such as silicon carbide (SiC), silicon nitride (Si3N4), composites having a SiC or Si3N4 matrix, silicon oxynitride, and silicon aluminum oxynitride; a Si-containing metal alloy, such as molybdenum-silicon alloys (e.g., MoSi2) and niobium-silicon alloys (e.g., NbSi2); and an oxide ceramic such as mullite-containing ceramics (e.g., a mullite matrix with ceramic fibers, such as alumina fibers, dispersed in the matrix). The substrate may comprise a matrix reinforced with ceramic fibers, whiskers, platelets, and chopped or continuous fibers.
Other features, details and advantages of the invention will be apparent from the attached drawings and detailed description that follows.